Core Web Vitals, down to the element that broke them
PageSpeed Insights tells you your CLS is 0.25. WebGrowthSignal tells you it's the ad slot above your article, rendered by <AdBanner>, with no reserved height — and gives you the CSS. Every page is traced in a throttled mobile browser that records which element moved, what pushed it and which component rendered it, while the cookieless pixel reports real visitors' CLS, LCP and INP with the selectors behind them.
What Core Web Vitals covers
- Layout-shift attribution: the culprit element, not just the elements that moved
- Images and next/image without dimensions
- Ad slots (AdSense, GPT) and embeds without reserved space
- Hydration swaps — useMediaQuery, isMounted, client-only renders
- Web-font reflow
- Lazy-loaded, unprioritized or late-discovered LCP images
- LCP phase breakdown: TTFB, load delay, load time, render delay
- Scripts blocking the main thread (Long Animation Frame attribution)
- Real-user p75 CLS, LCP and INP per page, with top offending selectors
- Visual overlay of every shift on a page screenshot
Why teams use it
The cause, not the symptom
The browser reports the elements that moved. We find the image that loaded, the slot that expanded or the component that re-rendered just above them a moment earlier — that's what you actually fix.
React-aware
Culprits are traced back through React's fiber tree to the component that rendered them, and hydration swaps from useMediaQuery-style hooks get their own diagnosis and fix.
Lab and field, side by side
A scripted scan catches problems before users do; the pixel shows which ones real visitors hit, on which pages, on which devices — and which selector to blame.
Core Web Vitals — frequently asked questions
- How is this different from PageSpeed Insights or Lighthouse?
- They measure the same metrics but report a score and generic advice. We keep the elements: each layout shift is matched to the image, ad slot, embed, font or React component that caused it, and each LCP and INP problem names the element or script responsible.
- Does the real-user monitoring need another script?
- No. The same cookieless pixel that powers Traffic and Heatmaps reports Core Web Vitals once per page view, using the browser's own performance APIs.
- Will component names be minified?
- On production builds usually yes — the report then falls back to CSS selectors and says so. Scanning a preview deployment gives full component names.
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